1. Field of the Invention
The field of the invention is data processing, or, more specifically, methods, systems, and products for determining availability of a destination for computer network communications.
2. Description of Related Art
An ICMP echo message is commonly used to determine availability of a destination for computer network communications. An ICMP echo reply message represents the fact that the destination is available for computer network communications. When many clients or sources on a network attempt to gather availability information about many destinations, each source may generate many ICMP echo messages, each destination may generate many ICMP echo reply messages, many routers may be in many data communications routes between the sources and the destinations, and network performance suffers. This situation is common enough so that some system administrators disable the ICMP service on routers under their jurisdiction. This is practice that causes still further problems.
Consider path MTU discovery, for example. Networks of an internet have different maximum packet sizes. Sometimes this fact can be administered by fragmenting packets too large for a particular link. Another way to administer the fact of differing maximum packet sizes is to determine what the maximum packet size that all networks between the source and destination can handle by sending large packets and receiving ICMP messages back from routers along the way regarding maximum packet sizes. This second process is called ‘path MTU discovery.’ Sources implementing path MTU discovery typically send IP packets with the ‘don't fragment’ bit set in the IP header. Then, when a router cannot deliver the packet to the next hop, it will return an ICMP ‘Destination Unreachable’ message to indicate that the packet cannot be processed. The MTU for the next network hop is encoded in this ICMP Destination Unreachable message, advising the originating source of the size the packet should be when it is re-sent. The source therefore can iteratively determine the largest packet size that can be sent without fragmentation to a destination.
This situation is common enough so that some system administrators disable the ICMP service on routers under their jurisdiction. This is practice that causes still further problems. A problem occurs when a system administrator of a router between the source and the destination has been disabled ICMP on the router. In this case, the source cannot know what to do. It never receives an acknowledgement for the message it sent, yet no ICMP Destination Unreachable packet is returned to inform it that something went wrong. In this situation, accurate path MTU discovery is practically impossible. For all these reasons, improvements are needed in determining availability of a destination for computer network communications.